fix exact optional argument diagnostic

[a-tarasyuk]

Fixes #4221

---

else if (
    message === Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1
    && exactOptionalPropertyTypes
    && getExactOptionalUnassignableProperties(source, target).length
) {
    message = Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1_with_exactOptionalPropertyTypes_Colon_true_Consider_adding_undefined_to_the_types_of_the_target_s_properties;
}

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a regression test and adjusts diagnostic selection so that, with exactOptionalPropertyTypes, argument assignability errors include a more specific message when optional properties are unassignable due to undefined.

Changes:

• Introduces a new compiler test case + baselines for exactOptionalPropertyTypes argument errors.
• Updates relation error reporting to emit a specialized diagnostic when exactOptionalPropertyTypes causes optional-property unassignability.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/exactOptionalPropertyTypesArgumentError.ts	New repro test asserting the argument error messaging under exactOptionalPropertyTypes.
testdata/baselines/reference/compiler/exactOptionalPropertyTypesArgumentError.types	Adds the expected type baseline for the new test.
testdata/baselines/reference/compiler/exactOptionalPropertyTypesArgumentError.symbols	Adds the expected symbol baseline for the new test.
testdata/baselines/reference/compiler/exactOptionalPropertyTypesArgumentError.errors.txt	Adds the expected error text baseline showing the specialized diagnostic.
internal/checker/relater.go	Emits a more specific diagnostic when exactOptionalPropertyTypes is enabled and optional-property unassignability is detected.

[a-tarasyuk]

@ahejlsberg Do we need to handle this case as Strada does, or is this a case where we're intentionally taking a different direction?

else if (
    message === Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1
    && exactOptionalPropertyTypes
    && getExactOptionalUnassignableProperties(source, target).length
) {
    message = Diagnostics.Argument_of_type_0_is_not_assignable_to_parameter_of_type_1_with_exactOptionalPropertyTypes_Colon_true_Consider_adding_undefined_to_the_types_of_the_target_s_properties;
}